This invention relates to trench-gate field-effect transistors in which an insulated trench-gate structure extends to an underlying source region from a drain region adjacent to a major surface of the semiconductor body. The invention also relates to methods of manufacturing such transistors.
Trench-gate field-effect transistors are known, comprising a semiconductor body into which an insulated gate electrode extends in a trench from a major surface of the body. The trench extends through a channel-accommodating portion of a body region of a first conductivity type between drain and source regions of an opposite second conductivity type. Usually the source region is adjacent to the said major surface, where it is electrically shorted to a part of the body region.
United States patent specification U.S. Pat. No. 5,134,448 discloses a trench-gate field-effect transistor of what may be termed an inverted configuration, in which the drain region is adjacent to the said major surface with the insulated gate electrode. In this case, the insulated gate electrode extends in a trench from a major surface of the body, successively through a drain region, a lower-doped drain drift region, and a transistor body region to reach an underlying source region of the transistor. A part of the body region is electrically shorted to the underlying source region by a buried electrical short. The whole contents of U.S. Pat. No. 5,134,448 are hereby incorporated herein as reference material.
U.S. Pat. No. 5,134,448 teaches burying the electrical short at the bottom of a trench, where it is formed by a variety of ohmic-contact means. Such means include a metal (for example, Al, Ti, W, Mo, Ta, Ni, Cr, Pt, or alloy thereof), an intermetallic with the semiconductor, and a degenerate semiconductor. The layout area of the device is significantly increased when an extra trench (separate from that of the trench-gate) is provided specifically for the electrical short. The trench-gate structure becomes complicated when the electrical short is provided at the same trench as the insulated gate electrode.
It is an aim of the present invention to provide a trench-gate transistor of inverted configuration, which can have a compact layout of drain and trench-gate structures at the body surface and a buried source-body region short that does not complicate the trench structure of the insulated gate electrode.
According to a first aspect of the invention, there is provided a trench-gate field-effect transistor of inverted configuration, in which the insulated gate electrode extends in a trench lined with gate dielectric that insulates the gate electrode from the drain region, a drain drift region, the transistor body region and the underlying source region. The gate dielectric is thicker adjacent to the drain region than adjacent to a channel-accommodating portion of the body region. A more highly doped bottom portion of the body region forms a leaky p-n junction with the underlying source region at an area that is separated laterally from the insulated gate electrode by an active portion of the source region adjacent to the trench. The leaky p-n junction provides the buried electrical short. The body region comprises an overlying layer that provides the channel-accommodating portion and that is less highly doped than the bottom portion. The active portion of the source region extends across the highly doped bottom portion of the body region to connect with the channel-accommodating portion of the body region adjacent to the trench.
Such an arrangement of the trench for the insulated gate electrode with respect to the various transistor regions permits a compact layout of the transistor. In particular a compact layout of drain and trench-gate structures is achievable at the body surface, and a compact layout of the buried electrical short is achievable with the underlying source region. Premature breakdown between gate and drain across the gate dielectric is avoided by making the gate dielectric to be thicker adjacent to the drain region than adjacent to the channel-accommodating portion of the body region. A thicker dielectric may also be advantageously provided adjacent to the lower-doped drain drift region, for example in a power device with cells which are so closely packed that the drift region is depleted by RESURF action from neighbouring trench-gate portions in the voltage blocking state of the device.
In order to permit the provision of the buried electrical short in a simple manner, it is advantageous for the highly doped bottom portion of the body region to be in the form of a layer that extends laterally to the gate trench where it is overdoped by the said active portion of the source region. In this case, the active portion of the source region can be formed by dopant implantation and/or diffusion. Thus, its doping concentration profile of the second conductivity type may be implanted at the bottom of a trench in the semiconductor body before the gate electrode is provided in the trench. Its final doping profile of the second conductivity type may correspond to a dopant diffusion profile from the bottom of the trench.
According to a second aspect of the invention there are also provided advantageous methods of manufacturing trench-gate field-effect transistors in accordance with the first aspect.
Some of the particularly advantageous technical features and some of the options available with the invention are summarized in the appended Claims.
The present invention is particularly advantageous for realising power devices with a compact transistor layout geometry. Such devices generally comprise a plurality of the body regions that are located side-by-side in the semiconductor body, with grid portions of the trench-gate structure in-between. The realisation of the electrical short in accordance with the present invention permits a close spacing of these grid portions, and even permits the active portion of the source region to be provided in a self-aligned manner around the bottom of each grid portion of the gate trench.
The drain drift region is lower doped than the drain region and so has a lower conductivity. In order to reduce the on-resistance of the device, it is advantageous for the drain drift region to have a doping concentration of the second conductivity type that increases towards the drain region. This can be readily achieved with the inverted configuration of the transistor by, for example implanting and/or diffusing dopant from the surface of the drain region and/or from the surface of the drift region before providing the drain region. Different doping profiles can easily be produced in this way for the drift region.